This invention relates to ultrasound surgery, and more particularly to a method and assembly for the controlled use of cavitation during diagnostic and therapeutic ultrasound procedures.
Chronic excessive bleeding of the endometrium of the uterus, termed endometriosis, is thought to have a genetic basis in many patients and is estimated to afflict 15% of women in their reproductive years. Heat cauterization of uterine blood vessels has been attempted as a means to treat endometriosis, wherein a hot fluid filled balloon is inserted through the cervix and then inflated to contact the uterine interior. However, this approach has trouble reaching all the internal folds of the uterus, and heat cauterization provides only partial and short-lived benefits. The predominant method for the treatment of endometriosis, as well as uterine fibroid tumors and related gynecological disorders involves the surgical removal of uterine cells or complete hysterectomy, both of which are invasive means of removing pathological uterine tissues. These methods leave scars or other disfigurements which can be both physically and psychologically debilitating.
Compared with other surgical methods, a primary advantage of ultrasound surgery is its noninvasive nature. Ultrasound allows diagnostic and therapeutic procedures to be accomplished either wholly from means external to the body, or with minimal dependence on procedures no more invasive than current laproscopic techniques. Being noninvasive, the cost advantages, both in hospital stay and in surgical preparation time, are readily apparent. In addition, the lack of cosmetic disfigurement and risk of infection are both significant advantages for ultrasound procedures.
Ultrasound can be utlized for diagnostic imaging, where an ultrasound transducer is used to generate ultrasonic waves which are directed at a region of interest in a patient. The transducer then receives reflected ultrasonic waves from the region and converts the received waves into electrical signals from which an image is generated. Ultrasound has also been used in various therapeutic applications. One such application, thermally-based ultrasound surgery, involves applying ultrasonic waves to a targeted treatment volume, such as a tumor, in order to heat the treatment volume and create a lesion. An example of such an application can be found in U.S. Pat. No. 5,694,936 issued to Fujimoto et al. Another application of therapeutic ultrasound is in the treatment of vascular thrombosis as seen, for example, in U.S. Pat. No. 5,648,098 issued to Porter. Unfortunately, the otherwise beneficial results of both diagnostic and therapeutic ultrasound procedures are often made unpredictable by the phenomenon of acoustic cavitation.
Acoustic cavitation is a term used to define the interaction of an acoustic field, such as an ultrasound field, with bodies containing gas and/or vapor. This term is used in reference to the production of small gas bubbles, or microbubbles, in the liquid. Specifically, when an acoustic field is propagated into a fluid, the stress induced by the negative pressure produced can cause the liquid to rupture, forming a void in the fluid which will contain vapor and/or gas. Acoustic cavitation also refers to the oscillation and/or collapse of microbubbles in response to the applied stress of the acoustic field.
The induced oscillation of microbubbles can generally be categorized as noninertial cavitation or as inertial cavitation. Noninertial cavitation appears at very low acoustic pressure amplitudes, as soon as microbubbles are present in a tissue. In noninertial cavitation, the walls of the microbubbles oscillate at the frequency of the ultrasound field generally without damaging surrounding cells, but considerably disturbing ultrasound transmission by reflecting or scattering incident waves. Inertial cavitation appears rather suddenly at higher incident pressures, defining a cavitation onset threshold. In inertial cavitation, microbubbles expand to reach a critical size after which the collapse is driven by the inertia of the surrounding fluid, thus the term xe2x80x9cinertialxe2x80x9d cavitation. Microbubble size is a determining factor in the degree of response to the ultrasound field, such that microbubbles are highly resonant oscillators at certain drive frequencies. Microbubble oscillation can be sufficiently violent to produce mechanical or thermal damage on surrounding tissue, thereby creating lesions.
In current practice, significant steps are usually taken to avoid cavitation, as described in U.S. Pat. No. 5,573,497 issued to Chapelon. Typically, cavitation is only permitted where it can be very carefully controlled and localized, such as at the end of a small probe or catheter as in U.S. Pat. No. 5,474,531 issued to Carter. The primary reason for avoiding cavitation is that thresholds for inducing cavitation of microbubbles are unpredictable due to the diversity of microbubble sizes and quantities in different tissues. Uncontrolled cavitation hinders the penetration of ultrasonic waves into tissue, and can lead to uncontrolled tissue destruction outside the intended treatment volume. As a result, surgical protocols have been formulated which attempt to increase cavitation onset thresholds in most diagnostic and therapeutic applications.
Cavitation occurs more easily at low frequencies of ultrasound transmission, with the cavitation threshold increasing as the frequency of ultrasonic waves is increased. Therefore, the predominant method of controlling cavitation during ultrasound procedures has been to utilize high frequency ultrasonic waves, as disclosed, for example, in U.S. Pat. No. 5,601,526 issued to Chapelon et al. and in U.S. Pat. No. 5,558,092 issued to Unger et al. However, this approach is not without drawbacks, as high frequency ultrasound cannot penetrate as far in soft tissue or through bone. In addition, high frequency ultrasound often has the detrimental effect of excessively heating tissues located between the ultrasound transducer and the intended treatment volume.
Contrary to past approaches, it is an object of the present invention to provide a method and assembly for performing ultrasound surgery which uses, instead of avoids, cavitation for diagnostic and therapeutic ultrasound procedures.
It is a further object of the present invention to provide a method and assembly for performing ultrasound surgery which makes cavitation thresholds predictable.
It is another object of the present invention to provide a method and assembly for performing ultrasound surgery which creates a controlled lesion within an intended treatment volume.
It is a further object of the present invention to provide a method and assembly for performing ultrasound surgery which utilizes low frequency ultrasonic waves to create the lesion.
It is another object of the present invention to provide a method and assembly for performing ultrasound surgery which allows for a preview of the expected lesion location within a treatment volume.
It is yet another object of the present invention to provide a method and assembly for performing ultrasound surgery which allows verification of proper lesion formation within the treatment volume.
It is a further object of the present invention to provide a method and assembly for performing ultrasound surgery in which substances associated with the microbubbles can be delivered within the treatment volume.
Accordingly, a method and assembly are provided which use cavitation induced by an ultrasound beam for creating a controlled surgical lesion in a selected treatment volume of a patient. First, a plurality of microbubbles are provided in the treatment volume. Preferably, the threshold for cavitation of microbubbles in the treatment volume is lowered compared with the threshold for cavitation in surrounding tissues. The expected location of the surgical lesion within the treatment volume may be previewed, and then the microbubbles in the treatment volume are cavitated with the ultrasound beam to create the controlled surgical lesion. In addition, substances such as drugs or DNA can be associated with the microbubbles such that cavitation of the microbubbles delivers the substances to the treatment volume. Preferably, the creation of the surgical lesion at the expected lesion location is then verified. Using the method and assembly of the present invention, the cavitation threshold within the treatment volume is made predictable, and a low frequency ultrasound beam may be used to cavitate the microbubbles within the treatment volume without causing damage to surrounding tissues.
The above objects and other objects, features, and advantages of the present invention are more readily understood from a review of the attached drawings and the accompanying specification and claims.